Basement Construction and Structural Design

Good basement construction starts with solid, strong soils. I
mentioned earlier that your foundation, or footings, must be placed on
solid ground, or what’s called undisturbed soil.
Undisturbed soil is soil that has been in place for thousands of years.

No one has dug it up and filled it back in. It was compacted by Mother
Nature with gravity. The soil particles have become tightly packed.
Undisturbed soil will not compact under the weight of the house. That’s
what you want when building a basement.

Your foundation should never settle.
Settlement in a house is a misconception. You often hear people refer to
settlement cracks in their house.

They have doors that stick and
drywall with cracks. They think their foundation has failed and believe
this happened because the foundation has settled into the earth. It
probably has not actually done that unless the basement construction was
very badly designed and built or the soils were not inspected or tested
before the footings were poured.

Settlement Cracks

The reason new houses
develop so called “settlement” cracks is actually because they actually
shrink. The wood joists, rafters, sill plates, and even the studs the
house is built of will shrink as they lose moisture. The wood shrinks
most in the direction perpendicular to the grain. The wood fibers move
closer together as the moisture leaves. Studs don’t shrink much along
their length since that direction is parallel to the grain of the wood.
But the plates and sills that lie horizontally on the top and bottom of
the studs, the floor joists, and the ceiling joists can shrink quite a
lot.

I did a calculation once and was shocked to discover that a two-story
frame house can shrink as much as two inches from top to bottom as the
wood dries to its ultimate moisture content. Drywall, doors, windows,
and other house components do not shrink that much. Because of the
differential shrinkage, you end up with sticky doors and drywall cracks.
You get gaps at mouldings and in floors. These cracks will grow and
shrink seasonally. But after the first winter heating season, things
should stabilize and you can patch these. Most of them will not return,
if your house was built right.

Sorry for the diversion. Now back to our discussion of basement
construction. If your house is built on solid soil, you will not get any
settlement of the foundation or basement walls. If you are building in
an area that you know or suspect has poor soil or unsuitable soils, you
will need the advice of a soils engineer, more properly called a geotechnical engineer.

He or she will test your soils and prescribe what has to be done to
make the soils strong enough to support your house for a long, long
time. It might require removal and replacement of the bad soil. It might
mean draining and compacting wet soils. But whatever you have to do,
when the amendment of the soil is complete, be sure to have the engineer
retest and certify that the soils are now good for construction.

Basement construction can be done with concrete block, poured concrete, pre-cast concrete, or even treated wood. I have not built a basement with treated wood walls and cannot comment on their benefits of shortcomings. But the International Residential Code has requirements and details about this type of construction.

Precast concrete foundation systems
can save money by reducing on-site costs. This type of basement
construction uses factory-made concrete panels that are shipped to the
site and assembled on site-built footings. many times the footings are
actually gravel trenches and not poured concrete. The cost saving come
from the elimination of the need to build concrete forms and then strip
away the forms and transport them off-site. But usually those benefits
are not significant for a “one of a kind” house. In houses that are
built repetitively in a subdivision, there is an economy of scale with
precast concrete foundations that can save both time and money that is
multiplied over many houses. Precast concrete basement construction is
quite strong. But it is not as commonly done as poured concrete or
concrete block. If you’re buying a house with this type of foundation,
you do not have to worry about that. Just be sure to get documented
engineering certification on the design. It will give you peace of mind
and also reassure a future buyer of the integrity of the basement and
house foundation.

Basement construction with concrete block is quite
common. It will produce a strong basement in most locales. Concrete
block tends to be the most economical way to build a basement. You'll
have many more concrete masons to bid the job than sub-contractors who
can do poured concrete basement construction. The competition tends to
produce lower prices. However, concrete block, even if reinforced, is
not appropriate for areas that have expansive soil. That’s soil that
swells a lot when it gets wet and shrinks when it dries. The swelling of
this soil will apply lateral loads (pressures) to your basement wall
(push on it sideways) and can actually cause the basement construction
to crack and fail. If this is your soil type, you’ll need reinforced
concrete basement walls. Again, the local
Building Code
or a geotechnical or soils engineer can help you determine this.

Although
not as strong as reinforced concrete basement construction, concrete
block basements are absolutely strong enough to support a house and can
be reinforced to make them even stronger if your site conditions require
it. The amount of reinforcement you need will depend on the amount of
soil your basement walls will hold back. A structural engineer can
advise you on the exact way to do this. But here are some basic
guidelines.

If the level of the soil outside your basement
(it’s called the finished grade) is higher than your basement floor, you
are said to have an “unbalanced backfill” condition If
the height of the unbalanced backfill is less than four feet, your
basement construction can be done with a simple unreinforced, 8” block
wall. If the unbalanced backfill is up to five feet, you’ll need a 12”
block wall. If it’s up to six feet, you’ll need a 12” concrete block
wall that is filled solid with grout or built with solid block units. If
the unbalanced fill is higher than that, you will need additional steel
reinforcing. Remember, these are merely rough rules of thumb. Your
local
Building Code
can give you specific requirements for basement construction in your
location. It's always a good idea to consult with a structural engineer
to make sure your foundation and basement are structurally sound and
comply with the codes.

For areas with expansive soils and
if your house will have an unbalanced fill condition above six feet,
reinforced concrete basement construction would be best. This is the
most expensive option. But consider how much you are investing in the
construction of your house. It is probably a prudent investment to make
sure your house foundation will perform as intended and not require
expensive repairs in the future. A structural engineer is necessary for
the design of a reinforced poured concrete foundation.

Reinforced poured concrete basement construction is a
multiple step operation. First, forms are erected according to the
structural or architectural plans. Next, reinforcing steel bars are
assembled within the forms and tied together. When that is completed,
concrete is poured into the forms. When the concrete is two or three
days old, the forms are stripped away and the concrete basement walls
remain. By then, the concrete is strong enough to support itself in that
short time. After seven days, it is nearly up to its ultimate strength.
But the concrete curing process will continue for several more weeks
until the chemical process within the concrete is complete.

There
is one aspect of poured concrete basement construction that actually
offsets some of the additional cost. Concrete foundations can be built
very quickly, sometimes in as little as a week. This speeds up the
construction schedule. Rain and foul weather can delay progress on a
concrete block basement. The time savings converts to money savings
you'll see in site supervision costs and construction financing.

One
additional benefit of poured concrete basement construction is that
concrete walls are intrinsically much more resistant to water
infiltration than concrete block walls. You still should waterproof the
basement walls properly. But the added water-tightness of poured
concrete will give you a second line of defense against damaging water
intrusion.